1. Field of the Invention
This invention relates to systems and methods for controlling process parameters for a xerographic printing machine.
2. Description of Related Art
The use of charging devices for photoreceptors in xerographic printing is well known in the art. Typically, such charging devices may be one or more of a corotron, a dicorotron, a pin corotron, a scorotron, a discorotron, and/or a pin scorotron. Such charging devices may include a chamber arranged with one or more charge-generating emitters such as, for example, a wire, a dielectric wire, or a pin array. Some charging devices may also include a control grid to regulate and control the charge provided to the photosensitive member. In this way, the photosensitive member may receive a uniform charge at a desired potential.
As known in the art, a key characteristic of a charging device is the di/dV ratio or charge generating emitter ratio of the charge-generating emitter of the charging device. The di/dV ratio is also known as the “slope” of the emitter, and is generally expressed in units of Amperes per volt-meter. Typically, charging devices having a high slope have high overshoot output voltage, i.e., generate a voltage on the photoreceptor that is above the grid voltage, and have poor charging uniformity. In a pin scorotron, the ions generated from the coronode are accelerated by the field force past the screen or grid to reach the photoreceptor surface, thus increasing the surface potential beyond the grid voltage.
When the surface potential reaches the same voltage as the voltage on the screen or grid, there is no electrostatic field between the screen and the photoreceptor. However, since the ions have high residual momentum as the ions approach the grid from the coronode side, the ions will continue to penetrate the grid and build up a space charge. This extra space charge drives some ions to the photoreceptor surface, increasing the surface potential further, until the repulsion field force is large enough to prevent further ion transport. The overshoot voltage may be defined as the extra difference in voltage, above the grid voltage, that the photoreceptor potential needs to reach to prevent further ion transport.
As the current flowing from each pin differs from pin to pin, the time to reach the final overshoot voltage also varies from pin to pin. The time required for charging a surface under the charging device is determined by the width of the charging device and the process speed of the photoreceptor surface being charged past the charging device. This time may be limited by practical considerations, and not all pins may reach the ultimate overshoot voltage. All of this tends to limit the voltage uniformity of practical pin devices.
However, uniform photoreceptor charging is required to achieve high-quality xerographic results. As such, various ways to achieve desired levels of uniform charging are known. For example, U.S. Pat. No. 6,459,873 to Song et al. discloses a DC pin scorotron charging apparatus for charging a photoreceptor to a desired voltage. In this charging device, a first DC pin scorotron charging device initially charges the photoreceptor to an intermediate overshoot voltage. A second DC pin scorotron charging device thereafter uniformly charges the photoreceptor to the final voltage. The first charging device provides a generally high percent open control grid area, a generally high emitter slope, and a generally high emitter pin current. The second charging device provides a generally low percent open control grid area, a generally low emitter slope, and a generally low emitter pin current.
The goal of the first charging device is to provide the majority of the charging ions to the photoreceptor. The first charging device is designed as a high slope device with high screen open area, high coronode voltage (current) and close pin-to-screen spacing. This design tends to result in high overshoot voltage. Therefore, the screen voltage is purposely set lower than the required charging voltage. An offset voltage is defined as the grid voltage difference between the first charging device and the second charging device. The offset voltage is important and should be greater than the overshoot voltage of the first charging device.
The second charging device provides “uniform” charge leveling with little charge-up needed to bring the entire voltage of the surface being charged to the desired photoreceptor potential as uniformly as possible. Because the first charging device has provided most of the charging ions to the photoreceptor, the first charging device significantly reduces the required charging capability of the second charging device. Thus, the second charging device may be a low slope, low overshoot device. This may be accomplished by decreasing the screen open area, for example, to less than 50–60 percent, lowering the coronode voltage (current) and/or increasing the pin-grid spacing in the second charging device relative to the first charging device. Because each of these changes may improve the charging uniformity of the second charging device relative to the first charging device, the final photoreceptor potential should be close to the applied screen voltage on the second charging device with little overshoot.